Method for producing calcined clay pigments

Abstract

Naturally-occurring hydrated kaolin clay is calcined under a reducing atmosphere at a temperature of at least 1400°F., preferably in the range of about 1600°F. to 2100°F., and then the clay is calcined in an oxidizing atmosphere at a temperature of at least 1400°F., preferably in the range of about 1600°F. to 2100°F., to produce a calcined clay pigment product of desired brightness.

I claim: 
     
       1. A method for producing a calcined kaolin clay pigment which comprises heating finely divided particles of hydrated crystalline kaolin clay in a calciner under a reducing atmosphere to a temperature in the range of about 1400°F. to 2200°F., the time and temperature of heating being sufficient to dehydrate the particles of said clay withou sintering them, and thereafter heating the clay in a calciner in the presence of sufficient air to provide an oxidizing atmosphere to a temperature at least as high as the temperature to which said clay particles were heated under the reducing atmosphere and below the temperature at which sintering takes place, the temperature and time for heating in the oxidizing atmosphere being sufficient to result in a calcined clay pigment product that is brighter than the original hydrated clay and is also brighter than the calcined clay would be if all of the heating had been carried out under the oxidizing atmosphere at the temperature used when heating under the oxidizing atmosphere. 
     
     
       2. The method of claim 1 wherein said clay is heated under said reducing atmosphere and under said oxidizing atmosphere to temperatures below 2200°F. 
     
     
       3. The method of claim 1 wherein the clay is heated under the reducing and oxidizing atmosphere to temperatures in the range of 1600°F. to 2100°F. 
     
     
       4. A method for producing a calcined kaolin clay pigment which comprises heating finely divided particles of hydrated crystalline kaolin containing carbon in a calciner to a temperature in the range of 1400°F. to 2000°F. in the presence of air limited in amount to provide a reducing atmosphere for from 10 to 60 minutes and thereafter heating the clay in a calciner in the presence of sufficient air to oxidize carbon and provide an oxidizing atmosphere to a temperature which is at least as high as the temperature to which the clay was heated under the reducing atmosphere and below 2200°F. for from 10 to 60 minutes, the temperature and time for heating in the oxidizing atmosphere being sufficient to produce a calcined clay pigment that is brighter than the hydrated clay and is also brighter than the calcined clay pigment that would be obtained by carrying out all of the heating in the oxidizing atmosphere at the temperature used in heating the clay in the oxidizing atmosphere. 
     
     
       5. The method of claim 4 wherein said clay is heated under said reducing atmosphere and said oxidizing atmosphere to temperatures in the range of 1600°F. to 2100°F. 
     
     
       6. The method of claim 4 wherein at least a portion of the carbon is present as an impurity in the clay. 
     
     
       7. The method of claim 4 wherein at least a portion of the carbon is present as an additive with the hydrated kaolin clay prior to heating said clay under the reducing atmosphere, the amount of carbon added being in amount in the range of 0.1 percent to 5 percent of the weight of the clay. 
     
     
       8. A method for producing a high brightness calcined clay pigment from finely divided hydrated kaolin clay containing a small amount of a ferruginous impurity and less than 0.1 percent carbon as an impurity which comprises adding to said clay a source of carbon in amount of at least 0.1 percent by weight of the dry clay weight, heating the clay containing the added carbon to a temperature in the range of 1400°F. to 2000°F. for 10 to 60 minutes while maintaining the clay under a reducing atmosphere, and then calcining the clay for from 10 to 60 minutes in an oxidizing atmosphere to a temperature in the range of 1600°F. to 2200°F., thereby producing a calcined clay pigment having a brightness of at least about 90 percent, said calcined clay being brighter than the original hydrated clay and also being brighter than the calcined clay would be if all of the heating had been carried out under the oxidizing atmosphere at the temperature used when heating under the oxidizing atmosphere. 
     
     
       9. The method of claim 8 wherein the source of carbon is charcoal. 
     
     
       10. The method of claim 8 wherein the source of carbon is coal. 
     
     
       11. The method of claim 8 which is carried out by calcining the clay containing added carbon in a closed vessel to provide an atmosphere containing insufficient oxygen to burn completely the carbon and thereafter the clay is calcined in an open vessel.